The last obstacle in the 10-mile climb to Sperry Glacier—a 4,800-foot elevation gain—is a rock stairway cut in the raw stone, called "The Staircase." The ordinary name underplays the severity of the ascent. It has the exposure and verticality of a fire escape on a New York skyscraper. The steps are set in a triangular corner, which reaches up to a lofty window in the last 200-foot-high bench below the glacier. Mercifully, a cable railing is anchored to the rock. My right hand clenches it until knuckles rub raw. But there's no alternative. It's the only way to the window at the top.

The Staircase is precarious in the best of conditions, but rain or snow makes the limestone slick and treacherous. Today, nature's cruel joke is the wind. A forty-mile-per-hour burst of air tears through the vertical chamber, seemingly from all sides, as if the skin were being flayed off the building. The rock wall must be ricocheting the breeze: It's hard to make out its origin. The wind makes me more careful. Every step—there are 30 to my count—requires hefting my foot cautiously and, with one hand on the cable and one hand on my trekking pole, rolling my gait upward into the dark stairwell. Halfway up, my legs feel as heavy as waterlogged timber. But each new step brings me closer to heaven: Sunshine bathes the top of the chute.

At the 8,000-foot summit, my comrades and I stand up and cheer. The loudest hoots and hollers come from our guides, Stacey Bengtson and Mike Davies. The other climbers are from Colorado. I stayed with them the night before at Sperry Chalet, a stone-and-wood lodge perched a couple of thousand feet below. We are booked there for the duration of the trip—three days in mid-August 2011, after a wet winter. USGS glacier scientist Dan Fagre and his team, on a faster circuit of two days up and back, would come shortly and stay in a tent. They were used to roughing it up here. Sperry is their benchmark glacier, the one they monitor without fail every year.

Dan Fagre and his team of two—Kevin Jacks, forty, and Erich Peitzsch, 33—arrive two days later with impressive packs of gear, impressive not so much for their bulk as for the lethal accessories tied on top: ice axes, crampons, ice screws, and ropes. It is a tight squeeze through the Staircase—like threading the eye of a needle. A snag would be dangerous, but the three men slip through and break free cleanly.

Dropping their heavy packs, the men rearrange their equipment, getting set to make their rounds. From the beginning, field notebooks at the ready, it is clear the diagnostic work will be nothing like that of Grinnell Glacier. Here, Fagre must listen to the glacier more closely.

But first: precautions. To combat the threat of hidden crevasses, the three men don climbing harnesses and tie up to a fifty-meter, blue-and-green perlon rope. They attach crampons to their boots and arm themselves with ice axes. In case one of them falls into a crevasse, each man has ascenders to climb the rope back up to his comrades. The scientists will navigate the snow with half a rope length between each man. It will be critical that the line is taut. Tension on the line will detect a fall quickly— like a quiver. If there is a fall, the second man will drive his axe into the ice deck, and the lower man will climb up the rope and out. That's if everything goes according to plan, but mountains sometimes have schemes of their own.

Before tackling the ice, the men are curious to check on their weather station, a remote listening post running since 2006. It sits on the prow of the rock rib just above the men, tethered to the rock. Resembling a lunar module, though even more compact, the station operates with only minor maintenance as long as the snow doesn't cover its solar panel. Snowfall can exceed thirty feet at Sperry. Thus, it shuts down each winter until late spring. But all summer long it senses and stores data on temperature, relative humidity, solar radiation, and wind. The solar panel recharges a car battery that runs the whole thing.

Fagre says, "We put the station here, carrying the works on our backs, to see if any weather conditions were different at this glacier. In a few more years, we'll have enough data to make a judgment. Maybe it's colder here than at similar places in the Park where there's no ice. Maybe it gets less sun. That would help explain why Sperry exists."

The men gather around the weather station, with their ropes still in coils over their shoulders. Their crampons clink against the rock. The wind gauge is whirring, and the weathervane is darting back and forth. Kevin drops to one knee, setting his ice axe down gently, to examine one of the guy wires to the station. It's loose. The wind is a continual threat to the equipment. Being covered by up to thirty feet of snow in winter doesn't phase the station, but wind could damage the works.

"Dan, I'll tighten this guy wire at the end of the day," says Kevin.

"Yeah, we should check all the sensors, too," says Fagre. "Looks like the battery is running."

"Good. I'm in no mood to lug another car battery up here."

"That's how you get a post with Dan," Erich says. "He sees how many car batteries you can carry, then maybe you get the job."

Kevin picks up his ice axe. He joins Fagre and Erich at the edge of the rock rib where it meets the ice and snow. They uncoil the rope and tie bowlines to clip into their harnesses. The two men look to Fagre for instructions.

Dan Fagre just points, unusual for a loquacious man. Ahead, about 200 yards into the glacier, toward the headwall, is a bald spot. The snow has melted away or been scoured by the wind to reveal a stretch of disturbed ice and crevasses about the size of a football field. The ice is dark and shiny, like the frozen film over a flooded parking lot.

The ice itself is exposed because the snow is thinner all around the ice patch. "The snow has been blown away and pillowed up elsewhere," Fagre says. Some curves along a glacier bowl can act like tunnels of rushing wind.

Those wind spouts have scoured at least five ice patches free of snow. Loosening up now, Fagre tells the men that they need to measure each patch to add up how much ice is exposed on the glacier. They will deduct this from Sperry's overall area (as figured last year) to calculate the percentage of snow cover. This is akin to measuring the net surface of a block of Swiss cheese: The holes don't count. At the end of summer, the snow coverage on a glacier is an indication of its health. Snow will convert into ice; enough of it and the glacier will expand. Anything around sixty percent coverage means the glacier is near equilibrium, holding its own. Over sixty percent typically means it's growing. A glacier accumulates most snow—and makes most ice—in its upper reaches. Most melting happens at the bottom.

For these reasons, snow cover usually drapes only the head of a glacier in late summer. This summer breaks that typical pattern: Snow runs all the way to the foot. This demonstrates the variability in mountain weather; still, on average recently, melting has outpaced the annual snowfall.

Fagre dispatches his crew toward the first ice patch. "Erich, you go first, my man," he says. Erich is the most experienced mountaineer, having cut his teeth on the crevasse-laden slopes of Mt. Hood (11,240 feet) while attending college in Oregon. Because of his skill, he is often the lead man on the rope. He has another advantage: He is skinny as a razor, the least likely of the three to fall in.

Once on the dark ice, Kevin belays Erich as he circumnavigates the patch. Erich records the ice's perimeter with his GPS equipment, making hundreds of readings that he'll plot back in the lab. Pacing off the ice patch takes forty minutes, but most of that is taken up by rope work. Each belay is a lifeline. Like a spoke, the rope plays out from hub to periphery. The team crisscrosses the oval ice as if spinning a web of silk.

Toward the end of the circuit, Kevin pays out the line to Erich so he can cross a snowbridge. The three-foot span connects two sides of a crevasse with the void looming underneath. The snow bridge likely formed when snowfall, adhering to the lips of the crevasse, spanned the gap. Meanwhile, an ice bridge represents an incomplete tear in the ice, as the glacier slowly moves.

Ice is fluid in a glacier and advances like brittle tar flowing downhill. The weight and pressure from the ice above makes it ooze. It slides and stretches and, when it moves faster, cracks, especially around steep corners or bumps in the road. The face of a glacier manifests what ever its rump is sitting on. Its countenance may be smooth or pitted, a gentle smile or a deep grin. The scale of the grimace is proportional to the speed of the ice and hence the steepness of the terrain underneath. When the ice slides over a hump of rock and falls precipitously down the other side, it cracks slightly; its surface looks like it has been cut into small ribbons.

But when the ice falls more rapidly down a steep, broad decline, the face fills with long, deep fissures—a crevasse field. On this scale, Erich is standing over a moderate disturbance, maybe a rock outcrop or two—a steeper slope than above. But it's more than a hundred feet beneath his toes. In between is a labyrinth of ice: It's possibly riddled with holes, and he's careful not to fall in. Gingerly, he steps off the bridge onto solid ice.

Securely on firm snow now, Erich and Kevin walk in step to the next ice patch, about one-quarter mile away. On two occasions, they must backtrack when snow bridges look dubious. Fagre trails, plotting in his mind how to survey the remaining four patches in record time. When they're finished, all five spots are measured in just over two hours—exceeding Fagre's time bud get by half an hour or so.

"Pacing off the holes in the snow cover took longer than gauging last year's snow line," says Fagre. "Tracking a snow line, halfway down the glacier, is like tracing the ruffles on a dress. One long wavy hemline—but pretty simple to track." And quick.

Today is like sewing buttons on a dress: There are several ice circles to complete. Time consuming. But he's got some flexibility in the schedule, as long as he gets off the glacier before dark. The GPS data, plotted back at the lab, will reveal that the snow covers ninety- seven percent of the glacier's area. Sperry's new acreage figure (206 acres) comes from last year's ice margins, measured when they were fully exposed. That year the snow line coverage fell under the sixty percent threshold, and the glacier was losing ice. Now, for two-thirds of the year, Sperry is growing—gaining ice so far. Next month or next season could bring more recession.

But for now, for the team, it's time to celebrate. "Yes," Fagre shouts, "it's a great summer for Sperry! Right now, the scales are tipped in its favor."

Here, in mid-August, Sperry is satiated, gobbling up the snow, a turnaround for a glacier that has seen major melt years over the past two decades, times when Sperry starved.

Those good years and bad years would even out if Sperry were stable—neither growing nor retreating over time. But, aside from minor blips like now, the record shows a steady retreat. The glacier is emaciated under this year's blanket of snow.